1. Field of the Invention
The present invention relates to the thermal treatment of irradiated propylene polymer material to render it stable on long-term storage in the presence of oxygen, and, more particularly, to an improved method of effecting the recombination of free radicals present in the irradiated material and, subsequent thereto, the deactivation of free radicals still remaining therein.
2. Description of the Prior Art
European patent application publication no. 190,889, published Aug. 13, 1986, describes high-molecular-weight, long-chain branched polypropylene made by irradiating linear polypropylene with high-energy ionizing radiation. The free-end branched polymer is gel-free and has strain-hardening elongational viscosity.
The process described in the aforementioned European publication for making the branched polymer comprises irradiating linear polypropylene with high-energy ionizing radiation in an oxygen-deprived environment, aging the irradiated material at about room temperature in the same type of environment for preferably about from 2 to 30 minutes, and then deactivating or quenching the residual free radicals by the application of heat or by the addition of an additive, e.g., methyl mercaptan, that functions as a free-radical trap. The purpose of the room-temperature aging is to allow partial recombination of the radicals to occur to produce long-chain branching. Thermal radical-deactivation methods disclosed in the above publication include a melt extrusion process described in the examples and a process utilizing a fluidized bed system.
In a process for irradiating polypropylene with the objective of producing long-chain branching, it is important to give particular attention to the fact that the radical recombination which is necessary for the production of branching occurs in a solid matrix and consequently is likely to be diffusion-limited. Another important consideration is the heterogeneous nature of the crystalline polypropylene of commerce, which actually is semi-crystalline, a typical crystalline/noncrystalline phase ratio by volume being about 60/40. A means is needed to effect the recombination of the free radicals produced in the polypropylene irradiation process, and finally the deactivation of residual free radicals, in a controlled manner so that predictable results can be achieved irrespective of the diffusion limitations and polymer heterogeneity mentioned above.